﻿<p>The aggregation relationship <em>IfcRelAggregates</em> is a special type of the general composition/decomposition (or whole/part) relationship
<em>IfcRelDecomposes</em>. The aggregation relationship can be applied to all subtypes of <em>IfcObjectDefinition</em>.</p>
<p>In cases of aggregation of physical elements into a physical aggregate the shape representation of the whole (within the same
representation identifier) can be taken from the sum of the shape representations of the parts.</p>

<blockquote class="example">
EXAMPLE&nbsp; A roof is the aggregation of the roof elements, such as roof slabs, rafters, purlins, etc. Within the
same representation identifier (such as the body geometric representation), the shape representation of the roof is given by
the shape representation of its parts.
</blockquote>

<p>Decompositions imply a dependency, implying that the whole depends on the definition of the parts and the parts depend
on the existence of the whole. The behaviour that is implied from the dependency relationship has to be established inside the applications.</p>

<blockquote class="history">
HISTORY&nbsp; New entity in IFC2x.
</blockquote>

<blockquote class="change-ifc2x4">
IFC4 CHANGE The attributes <em>RelatingObject</em> and <em>RelatedObjects</em> are demoted from the supertype <em>IfcRelDecomposes</em>.
</blockquote>

